building '/nix/store/qbr6qqs9gi96r5dpamalb76725zyyvj3-bootstrap-tools.cpio.bz2.drv'...
building '/nix/store/mswwp45p6pzbb3zv604h3li6lsm43l6s-bzip2.drv'...
building '/nix/store/fn6dmhxmki9ppkfgibr054z65sqxnw34-cpio.drv'...
building '/nix/store/5pgkcx95qbbjsdk04hxngqc1fnaxgjgf-mkdir.drv'...
building '/nix/store/ycjc52zyp25h42br5ikz0sip84vfynh8-sh.drv'...
objc[36265]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called.
objc[36265]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
objc[36267]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called.
objc[36267]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
objc[36269]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called.
objc[36269]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
objc[36285]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called.
objc[36285]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
objc[36317]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called.
objc[36317]: +[__NSPlaceholderArray initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
builder for '/nix/store/qbr6qqs9gi96r5dpamalb76725zyyvj3-bootstrap-tools.cpio.bz2.drv' failed due to signal 6 (Abort trap: 6)
builder for '/nix/store/mswwp45p6pzbb3zv604h3li6lsm43l6s-bzip2.drv' failed due to signal 6 (Abort trap: 6)
builder for '/nix/store/fn6dmhxmki9ppkfgibr054z65sqxnw34-cpio.drv' failed due to signal 6 (Abort trap: 6)
builder for '/nix/store/5pgkcx95qbbjsdk04hxngqc1fnaxgjgf-mkdir.drv' failed due to signal 6 (Abort trap: 6)
builder for '/nix/store/ycjc52zyp25h42br5ikz0sip84vfynh8-sh.drv' failed due to signal 6 (Abort trap: 6)

 - system: `"x86_64-darwin"`
 - host os: `Darwin 18.0.0, macOS 10.14`
 - multi-user?: `no`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.1pre6148_a4aac7f`
 - channels(root): `"nixpkgs-18.09pre144939.14a9ca27e69"`
 - nixpkgs: `/Users/jackhenahan/.nix-defexpr/nixpkgs`

It claims that I don't have a multi-user install and that sanboxing is on, but neither of those is true. :shrug:

export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES